Change attach_inline_image default domain from hostname to "inline".

This avoids problems with ESPs that don't distinguish *Content-ID*
from attachment filename, where a local hostname ending in ".com" could
cause Gmail to block messages sent with inline attachments.
(Mailgun, Mailjet, Mandrill and SparkPost have APIs affected by this.)

Fixes #112.

docs/sending/anymail_additions.rst

@@ -398,9 +398,15 @@ classes.)
     `idstring` and `domain` are optional, and are passed to Python's
     :func:`~email.utils.make_msgid` to generate the :mailheader:`Content-ID`.
     Generally the defaults should be fine.
-    (But be aware the default `domain` can leak your server's local hostname
-    in the resulting email.)
 
+    .. versionchanged:: 4.0
+
+        If you don't supply a `domain`, Anymail will use the simple string "inline"
+        rather than :func:`~email.utils.make_msgid`'s default local hostname. This
+        avoids a problem with ESPs that confuse :mailheader:`Content-ID` and attachment
+        filename: if your local server's hostname ends in ".com", Gmail could block
+        messages with inline attachments generated by earlier Anymail versions and sent
+        through these ESPs.
 
 .. function:: attach_inline_image(message, content, filename=None, subtype=None, idstring="img", domain=None)